A COMPRESSIONAL STUDY OF MGSIO3 ORTHOENSTATITE UP TO 8.5-GPA

A single crystal of MgSiO3 orthoenstatite has been studied by single-c rystal X-ray diffraction in a diamond-anvil cell up to a pressure of 8 .5 GPa. From the unit-cell data, it has been shown that the volume var iation with pressure is best described by two independent equations of state, with significantly different values for their room-pressure bu lk modulus, K0, and its first derivative, K0': at P < 4 GPa, K0 = 95.8 (3.0) GPa, and K0' -14.9(2.0); at P > 4 GPa, K0 = 122.8(16.5) GPa, and K0' = 5.6(2.9). A series of structural refinements carried out at pre ssure intervals of approximately 1 GPa shows that there is a change in compression mechanism at about 4 GPa that would account for this brea k in the equation of state of the pyroxene. Below 4 GPa the SiO4 tetra hedra are essentially incompressible, with no change in Si-O bond leng ths and O-Si-O bond angles, whereas at higher pressures the Si-O bond lengths shorten in a regular way, with no angular distortion of the Si O4 tetrahedra. The linear volume compressibility of both of the tetrah edra between 4 and 8 GPa is 0.0062(1) GPa-1 [corresponding to a bulk m odulus of 162(3) GPa]. By contrast, the Mg-O bond lengths decrease ste adily over the whole pressure range studied; the compression of each o f the MgO6 octahedra may be described by a single third-order Birch-Mu rnaghan equation of state: for the M1 site, K0 = 53.2(5.4) GPa, and K0 ' = 31.9(6.3); for the M2 site, K0 = 63.1(8.5) GPa, and K0' = 26.7(7.9 ). There is a well-defined change in both the degree of kinking of the tetrahedral chains (as measured by changes in the O3-O3-O3 chain exte nsion angle) and the amount of tetrahedral tilt toward the (100) plane at about 4 GPa. At low pressures the B chain, which is more distorted at room pressure, kinks dramatically while keeping the bases of its t etrahedra at a constant orientation of approximately 7-degrees from th e (100) plane, whereas above approximately 4 GPa the kinking ceases, a nd the tetrahedra begin to tilt steadily toward the (100) plane. The A chain behaves essentially in the reverse way: below 4 GPa the tetrahe dra tilt markedly toward the (100) plane with only a little chain kink ing occurring, whereas at higher pressures, the tetrahedral tilt virtu ally stops, and the kinking continues slowly in the opposite direction .